1. Field of the Invention
The present invention relates to the detection of defects in films and protective barriers. More specifically, it relates to the detection of defects in a dielectric layer, such as a film or protective barrier.
2. Background Description
There are various applications in which the presence of small defects in a dielectric layer is not acceptable. For example, in the case of dielectric substrates that are used as a base for the coating of chemicals used in photography or used the production of storage media, the presence of small xe2x80x9cpinholexe2x80x9d defects can lead to the defective coatings. In another application where the dielectric layer is used as a barrier material to ensure protection against biological particles (such as viruses) and toxic materials, the presence of defects, such as pinholes or tears, renders the barrier permeable to biological particles and toxic materials and inoperable as a protective barrier. In food packaging, when thin dielectric layers are used for packaging, the presence of pinholes can lead to food spoilage or contamination. For the above examples, as well as other situations, techniques to inspect dielectric layers for pinhole detection are needed.
The nature and size of the defects varies with the manufacturing methods used to produce the dielectric layer. For example, in the production of surgical gloves and other prophylactic items, a mold or mandrel is dipped into liquid stabilized latex. Defects are likely to be pinholes are tears. In the production of the base layer for photographic applications, physical anomalies arising from undisolved, unmelted clumps of material can serve as initiator sites for fractures in the extruding phase of the process. These fractures represent a source of defects. Both of the above type of defects will be hereinafter referred to as xe2x80x9cpinholesxe2x80x9d.
The defects or pinholes in cases of interest can be smaller than those detectable by optical inspection methods. Several approaches have been proposed for testing protective barriers in order to detect defects such as pinholes or tears. In U.S. Pat. No. 5,196,799 (Beard et al., issued on Mar. 23, 1993) a method for testing protective barriers, such as surgical gloves and other prophylactic items, is disclosed. The method disclosed in U.S. Pat. No. 5,196,799 comprises measuring the resistive and reactivity impedance of the protective barrier in order to determine if defects are present.
In U.S. Pat. No. 6,204,669 (Beard et al., issued on Mar. 20, 2001), another method of detecting defects in barrier material is disclosed. Placing a conductive liquid or gas on one side of the protective barrier or layer and an ionized gas (plasma) on the other, changes in the conductivity of the liquid or gas caused by ionized gas particles that pass through the defects can be monitored as disclosed in U.S. Pat. No. 6,204,669. The methods of U.S. Pat. Nos. 5,196,799 and 6,204,669 monitor changes in bulk properties such as impedance and, therefore, can not provide information regarding the characteristics of the defects.
In two published Patent Abstracts of Japan, Publication No. 10-123100 (Yukitoshi et al., published on May 15, 1998) and No. 11-218523 (Hirosaku et al., published on Aug. 10, 1999), methods are described for detecting pinholes in insulating films. In 10-123100, in order to test a layer for pinholes, the dielectric layer is placed on one electrode and another electrode is placed in contact with the top surface of the layer. A high-voltage is applied between the two electrodes. Since the electrodes contact the layer, only slow relative movements of the layer are possible.
In Japanese Patent Publication No. 11-218523, another apparatus for detecting pinholes in a dielectric layer is described. The layer is placed on a roll electrode that also serves to transport the layer. A second electrode is brought close to or into contact with the top surface of the layer. When the layer is electrified, a signal is detected. In this invention, the two electrodes are also in close contact with the layer thereby limiting the transport speed of the layer.
The presently available systems for detecting pinhole defects in a dielectric layer do not provide the ability to transport the layer at arbitrary speeds while at the same time being able to observe the location and characteristics of the pinholes.
It is the primary object of this invention to provide a system for detecting pinhole defects in a dielectric layer that allows the transport of the layer at arbitrary speeds while at the same time allowing the determination of the location and characteristics of the pinholes.
To achieve this and other objects, a system for detecting defects in a layer of dielectric material, the defects being characterized by the absence of the dielectric material where the absence of dielectric material provides a continuous path across the layer, is disclosed. The disclosed system comprises a first electrode, on which the layer is placed, at least one second electrode oppositely spaced apart from the first electrode and oppositely spaced apart from, but not in contact with, the top surface of the layer. A voltage, that is at least equal to the breakdown voltage corresponding to the spacing between the first and second electrodes in the absence of the material layer, is applied across the first and second electrodes. The presence and characteristics, such as the location and the cross sectional area, of defects are detected as a function of the flow of electrical current from the first electrode to the at least one second electrode. In one embodiment, a resistor is connected in series with the voltage source and either the first electrode or the second electrode and the applied voltage is a DC (direct current) voltage. One embodiment of the means for detecting the presence and characteristics of defects comprises a gas discharge bulb connected in series between the resistor and the electrode to which the resistor was connected, and a photodetector receiving optical radiation emitted by the gas discharge bulb. The layer can be a continues web of a given width; in that embodiment, the first electrode is a metal cylindrical roll electrode having a roll width at least equal to the web width; means of transporting the web allow the translation of the web; and the second electrode is a brush electrode or a linear array of brush electrodes having a width substantially equal to the web width.
The system of this invention, in the embodiment in which the first electrode is a metal cylindrical electrode, can be utilized to inspect webs of dielectric material for pinhole defects in applications such as photographic film, packaging material, base material for magnetic storage media, dielectric wrap for preservation of food, and the general purpose dielectric wrap. In the embodiment in which the first electrode is a flat electrode, the system of this invention can be utilized to inspect dielectric layer such as those used in batteries, for example. In another embodiment in which the first electrode is a holder mandrel for surgical or protective gloves or in which the first electrode is a holder mandrel for other prophylactic items, the system of this invention can be utilized to inspect these protective or prophylactic items. Another applications of the system of this invention is the inspection of dielectric layers used in food packaging. While the above description refers to pinhole defects, it should apparent that larger defects can also be detected. It should also be apparent that the system of this invention can be utilized for inspection and defect identification in other applications in which a dielectric layer is used.